1. Field of the Invention
This invention relates generally to the field of silica materials and, more particularly, to a process for converting a vitreous silica surface to the more stable form of cristobalite.
2. Description of the Prior Art
Silica, or silicon dioxide (SiO.sub.2), occurs abundantly in nature, comprising approximately 12 percent of all rocks, and exists in a variety of forms, both crystalline and amorphous. Silica is polymorphic (i.e. capable of existing in two or more crystalline forms), and the main forms of crystalline silica are quartz, tridymite, and cristobalite, which may be converted from one form to another at a given transition temperature. For example, quartz is converted to tridymite at 870.degree. C., and tridymite is converted to .beta.-cristobalite at 1470.degree. C. In addition, each of these three main forms of silica exists in a low-temperature modification (the .alpha.-form) and a high-temperature modification (the .beta.-form), which transform from one to the other at a given temperature. Crystalline silica is useful for, among other things, forming optical elements and components.
Vitreous silica, on the other hand, is in the amorphous form, in which the solid is not crystallized, due to the irregular arrangement of the atoms in the solid. Vitreous silica is widely used for making chemical glassware, in which various chemical processes and reactions are conducted. In particular, vitreous silica is used as a crucible to contain a melt, such as molten gallium arsenide, silicon, or metal sulfides, during the crystal growth of these materials from the melt. However, such processes typically occur at elevated temperature (e.g. 1240.degree. C. for gallium arsenide or 1410.degree. C. for silicon), and the vitreous silica is soluble in the melt to some degree at these temperatures. Thus, the vitreous silica crucible can contaminate the crystal growth melt, and the crystal grown from this melt can become contaminated with silicon and/or oxygen. It is the alleviation of this prior art problem of the undesirable surface properties of vitreous silica at relatively high temperatures to which the present invention is directed.